This application will explore the roles of extrinsic factors derived from bone marrow stromal cells in inducing the onset of immunoglobulin gene rearrangement. Specifically, the roles of interleukin-7 (IL-7), Flt2 ligand, (FL), and stromal derived factor-1 (SDF-1) will be studied. In Specific Aim 1, the ability of IL-7, FL, and SDF-1 to induce Ig recombination will be studied. Subsequently, experiments will be performed to determine the role of the receptors for these factors in participating in the pathway. Recombination requires both induced expression of the recombinase genes, RAG-1 and RAG-2, as well as accessibility of the immunoglobulin loci. In Specific Aim 3, the ability of IL-7, FL, or SDF-1 to stimulate RAG gene transcription will be studied. Finally, in Specific Aim 4 the ability of IL-7, FL, or SDF-1 to stimulate Ig heavy chain germline transcripts and heavy chain enhancer activity will be studied. Many of the experiments will use a recombinase substrate reporter vector that can express various mutants of the green florescent protein. Through the use of 7 color FACS analyses the recombinase activity and cell surface phenotypes of individual cells can be determined.